Is the whispering that comes to me during and after prayer, which adds an "alif" to "Allahu Akbar" making it "Aallahu Akbar," a form of doubt? And is it valid to continue the prayer while experiencing this whispering and ignoring it?

You have set foot on the path of correct treatment, so continue to repel the whisperings and turn away from them, not paying them any heed. Do not care about the thoughts that occur within you, for they do not invalidate your prayer nor do they cause you to leave Islam. Therefore, turn away from the whisperings concerning takbir, apostasy, and other matters.